This application is a divisional of application 10/152,873, filed on May 21, 2002, which issued as U.S. Pat. No. 6,881,838 on Apr. 19, 2005.
1. Field of the Invention
The present invention relates generally to recovering a polysaccharide gum from an aqueous fluid in which the gum is dissolved. More particularly, it relates to methods involving concentration, heat-treatment, and precipitation of a polysaccharide gum, and to polysaccharide gums recovered using such methods.
2. Description of Related Art
Certain polysaccharide gums (e.g., xanthan gum), which are hydrocolloids, are used in food, pharmaceutical, industrial, and oil field applications, because of their rheology. Addition of certain polysaccharide gums in foods can aid in making water-based products thicker, creamier, more visually appealing, and more stable over a wider range of temperature, pH and time. In non-food related applications, certain polysaccharide gums can provide viscosity for suspension, improved stability, and/or thickening of fluids. For example, in oil and gas drilling, certain polysaccharide gums can be used to modify the rheology of a fluid and to enhance the efficiency of drilling, workover, and completion operations.
Certain useful polysaccharide gums are extracted from plant material. For example, carrageenan is extracted from certain species of the class Rhodophyceae (red seaweed). Microorganisms can produce certain polysaccharide gums. Xanthan gum, for example, is a polysaccharide that can be produced by fermentation using bacterium of the Xanthomonas sp. Gellan gum is the generic name of a polysaccharide that can be produced by cultured Pseudomonas elodea or related organisms. Curdlan is a polysaccharide gum that can be produced by a microorganism (e.g., Alcaligenes faecalis varmyxogenes). Still other polysaccharide gums can be produced either by extraction from plant material or by microbial fermentation. Alginate is a polysaccharide gum that can be obtained by extraction from certain species of seaweed, or alternatively Azotobacter vinelandii or Pseudomonas aeruginosa can be used to produce the polysaccharide gum through fermentation.
Certain processes for the recovery and purification of polysaccharide gums (such as alginate, carrageenan, xanthan, pectin, gellan, welan, pullulan, curdlan, rhamsan, and sphingan polymers) from an aqueous fluid in which they are dissolved rely on a water-miscible non-solvent precipitation of the gum, such as an alcohol precipitation, among others. The aqueous fluid can comprise polysaccharides that are the products of microbial fermentation or extraction from a plant material. For example, isopropanol, ethanol, or acetone (e.g., water-miscible non-solvents of xanthan gum) can be used to precipitate xanthan gum from aqueous solution. In certain cases, the addition of salt(s) or adjustment of pH can reduce the amount of non-solvent required for precipitation of a polysaccharide gum. Following precipitation the gum can be dewatered and dried.
Recovery of a polysaccharide gum can be a difficult and expensive process, because polysaccharide gums can be viscous even at low concentrations, making handling difficult. Thus, mixing of reagents with the gum can be a power intensive process. The relatively low concentration of gum in fermentation broth (often less than about 5%) and in extracts from plant materials, as well as the high cost of certain gum non-solvents, and the losses associated with distillation of the gum non-solvent significantly impact the cost of processing polysaccharide gums.
Research to improve processes for the recovery of polysaccharide gums dissolved in aqueous fluid has focused on methods of reducing the amount of gum non-solvent needed to precipitate gum. Such methods have involved:                1. increasing gum concentration in an aqueous fluid comprising dissolved polysaccharide gum prior to non-solvent addition (e.g., via improved fermentation/extraction methods that achieve higher yield and concentration of the polysaccharide or via concentrating the extracts or fermentation products);        2. adding multivalent cations to an aqueous fluid comprising certain dissolved polysaccharide gums in order to increase precipitation efficiency, and reduce the amount of non-solvent that must be added; and        3. in addition to adding multivalent cations to an aqueous fluid comprising certain dissolved polysaccharide gums, adjusting its pH in order to increase precipitation efficiency, and reduce the amount of non-solvent that must be added.Such methods have their drawbacks. As stated above, it can be difficult to work with concentrated polysaccharide gum solutions, because even at relatively low concentrations of polysaccharide gum (less than about 5%), the solutions can be extremely viscous and difficult to handle. It can be difficult to remove biomass from polysaccharide gum fluids having relatively high concentrations of the polysaccharide gum that are obtained through methods involving fermentation processes. Furthermore, addition of cations and pH adjustment can result in the production of polysaccharide gum products (e.g., salts of polysaccharide gums) that have reduced solubility. Therefore, there is a long-standing need for improved processes for recovering polysaccharide gums.        